This is very cool. I've never used this before. I used to grind and fit and grind and fit etc. until bought a JD squared beast. Thanks for sharing your progress Brett, new and experienced builders will benefit from your build.
The reason I built the one piece adapter instead of stacking components on top of the orginal case cover is because I was concerned about maintaing concentricity between the gearbox and the engine output shaft. As on the adapter that Todd was making for the 3 cylinder engine. A one piece billet design also has more rigidity then a stacked component design and thats not my opinion its fact. The stacked design relies on the bolt tension to maintain rigidity. I chose the Rotax C gearbox because of it long reputation with using it on alternate engines. I personally built a turbo charged Honda long Ez and used a C gearbox. I flew it for 400 hours before I sold it. I couldnt find anybody that had gearbox failures because they couldnt handle the power i however did find 2 occurances of C gearbox failures because the adapter did not hold the gearbox rigid to the engine and the alignment got off (concentricity). Steve Henry did have a bearing failure with the c box and my adapter but after further investigation it appears it might have been a damaged bearing. He has over 150 hours sInce then on A c gearbox now flying with 180hp on take off using nitrous. As far as the Rk400 clutch Im not sure why someone would need to tear it down every 75 hours. Where did this information come from?
Our Avid speed wing project flew great yestarday. just a quick flight around the pattern but initial numbers are impressive. need more test flights to validate climb rates and such but it looks really promising
What you dont see in this cut away view is an additional mount that bolts to the cylinder head utilizing the two existing 6mm bolts. This is for the torque moment i think that you are referring too. I am shifting focus to the Apex engine since I have a good solution for the RX1.
I'm glad you brought this to the attention of nlappos, In this particular case the engineers at Rotax are offering to use their product with or without this option and a fellow builder is just getting info on how others have removed this optional product.
I'm sorry I miss read the original posting about this being a 4 stroke engine. This must be from a seadoo spark. I am unfamiliar with those engines. they may have a internal water pump but mounting a gearbox may be a choir. But if it is a good engine it just takes 1 person to do the homework then we can all benefit. lets us know what you find.
Those engine should be inline twins similar to the 582 or 618 series. They pose many challenges to convert them for aircraft use. For starters unless it is a newer 4 stroke seadoo they do not have internal water pumps, they instead use the pulpulsion jet to pump lake water through the cooling system. They do not have mounting provisions for a gearbox so a adapter arrangement would have to be made. The cost of trying to convert would far exceed the cost of just buying a 618 set up. I converted a HONDA aquatrax engine for use in a Long EZ aircraft that worked really well but it was a lot of work. The out come was not a 90 horse 2 stroke though. Its was a 120 horse inline 4 cyl. 4 stroke that was turbo normalized. I could develop 120 hp past 18,000ft.
The pulses per rev Has nothing to do with the number of cyl. Or number of cycles it fires at. It is only the number of electric pulses the ignition or lighting coil puts out per rotation. I believe this engine uses the lighting coils pulses which is probably 6 lighting coils.
You can pick what ever pitch you want. Simply pick the gear ratio you want out of the 4 ratio's offered for the c-gearbox. They are 2.62, 3 , 3.47 and 4:1. Most are using the 3:1 but Steve Henry used the 3.47 for a bit. He said he got the best STOL performance with it. He said it felt like the same power as his big bore 912 when he injected the nitrous. As far as longevity of these higher RPM engines that is a yet to be determined. I dont think there is enough flying hours on them to giess at a TBO #.
Just a word on your comment about the last bit of throttle not doing anything. Your kind of running the engine at a low max RPM. When you do this the throttle response changes. you might find that by increasing your rpm another 1000 (flatten the pitch) your throttle will behave a little different and you may even get better full economy. I know it sound counter intuitive. remember these engines develop their max HP at just above 10,000 RPM and the snowmobile racers are running them up to 11,000 RPM frequently.
I think the this biggest factor as far as starting is the prop diameter and moment of inertia. I don't have any problems starting on my trikes but Im running 68-70" 3 blade light propellers. I usually start with the enricher (choke) open and the throttle cracked open. When it fires I throttle up a bit and close the enrichener. Most important thing to remember, if it ever fires up but wont get past a really low idle just shut it down because it will just shake and idle really low. Enough shaking to possibly knock your airbox off or damage of stuff eventually.
One thing to note: the Yamaha engines do have an oil cooler it is just built into the side of the engine next to the oil filter. It is a liguid to liquid heat exchanger and it uses the engines coolant.